1. Field of the Invention
This invention relates to a resin composition for use in an electrophotographic toner which serves to develop an electrostatic image in electrophotography, electrostatic recording, electrostatic printing and the like.
2. Description of the Related Art
Conventionally, it has been proposed in Japanese Patent Laid-Open No. 134861/'80 to use a polymer containing an acid component in a toner composition for the purpose of improving its fixability. However, a toner using such a polymer is subject to the influence of environmental variations in that insufficient electric charges tend to be produced under high-humidity conditions and excessive electric charges tend to be produced under low-humidity conditions. Moreover, this toner is also undesirable from the viewpoint of triboelectric charging properties in that it is liable to fogging and suffers from a low image density.
Moreover, several applications of a polymer containing an acid anhydride in order to solve this problem are disclosed in Japanese Patent Laid-Open Nos. 123856/'86 and 280758/'87. In such a polymer containing an acid anhydride as a constituent unit, however, the acid anhydride is not uniformly dispersed in the polymer. Thus, the toner particles cannot be evenly charged, resulting in liability to fogging and poor developability.
Furthermore, in order to compensate for the undesirable properties of acid anhydrides, Japanese Patent Laid-Open No. 181263/'92 (EP 427275A) discloses a technique for preparing a toner having excellent triboelectric charging properties in which a polymer is formed by using an acid anhydride and an unsaturated carboxylic acid as copolymerizable monomers in such proportions as to give a specific acid value. However, since this technique uses a magnetic powder, the disposal of any discarded toner is troublesome. Moreover, since no technical measure is taken against the offset phenomenon and the acid monomers have poor dispersibility, this technique has the disadvantage that the carrier is liable to contamination when used in two components system, and the components used to improve triboelectric charging properties does not work effectively.
Furthermore, a charge controlling agent (CCA) has been used to improve the quantity of electric charge and triboelectric charging speed of toners. However, many of them contain a heavy metal and are highly toxic, and they are expensive. For these reasons, there is a recent tendency to reduce the amount of CCA added to the toner or replace it by a charge controlling agent consisting essentially of a resin (referred to as CCR). For example, Japanese Patent Laid-Open No. 15858/'91 discloses a process for the preparation of a polymeric toner using, as binder resin, a polymer composed of a styrene monomer and 2-acrylamido-2-methylpropanesulfonic acid and having a M.sub.w of 2,000 to 15,000. Japanese Patent Laid-Open No. 56974/'91 discloses a technique for the preparation of a negatively charged toner using, as binder resin, a polymer composed of a styrene monomer, 2-acrylamido-2-methylpropanesulfonic acid and other specific monomers and having a M.sub.w of 2,000 to 15,000. Japanese Patent Laid-Open No. 257461/'91 discloses a technique for the preparation of a magnetic toner containing a charge controlling agent comprising a polymer having a low molecular weight of 1,000 to 20,000.
In these techniques, however, the amount of charge controlling agent used cannot be reduced sufficiently and, moreover, other properties desired for toners cannot be achieved satisfactorily.